Stroke is a sudden loss of brain function resulting from interference with the blood supply to the central nervous system leading to cerebral ischemia. Many factors play a role in the development of brain damage after ischemia. Among these factors, oxidative stress has been shown to play a central role in cerebral ischemia. Oxidative stress is often initiated and propagated by overproduction of O2− and H2O2 and their conversion to potent oxidants, such as hydroxyl radical and perioxynitrate. In general, free radical production is low, and the organism can neutralize or metabolize the toxic effects by free radical scavengers such as super oxide dismutase (SOD) and catalase (Fridovich (1983) Annu. Rev. Pharmacol. Toxicol. 23:239-57). Nevertheless, under some pathophysiological conditions, there is oxygen radical accumulation that impairs the cells, such as free radical accumulation during cerebral ischemia (Ste-Marie, et al. (2000) Can. J. Neurol. Sci. 27:152-9; Mori, et al. (1999) Brain Res. 816:350-7). Furthermore, the burst of free radical production has been demonstrated at the onset of reperfusion after cerebral ischemia (Dirnagl, et al. (1995) J. Cereb. Blood Flow Metab. 15:929-40; Kumura, et al. (1996) Am. J. Physiol. 270:C748-52) . The brain is very susceptible to oxidative stress-induced damage because it is rich in polyunsaturated fatty acids and relatively low levels of endogenous antioxidant enzymes in neuronal tissue to neutralize the effect of free radicals (Juurlink and Sweeney (1997) Neurosci. Biobehav. Rev. 21:121-8). It has been demonstrated that free radicals and related reactive oxygen species mediate much of the damage that occurs after transient brain ischemia (Love (1999) Brain Pathol. 9:119-31). Moreover, SOD and glutathione peroxidase activities are significantly lower in stroke patients compared to control subjects, suggesting a significant correlation with infarct size, initial stroke severity and poor short-term prognosis (Demirkaya, et al. (2001) Eur. J. Neurol. 8:43-51). These data suggest that superoxide anion, O2−, hydrogen peroxide, and perioxynitrite are among the key reactive oxygen species implicated in ischemic injury.
Natural scavenger enzymes SOD and catalase are characterized by a very high efficiency and great stability toward oxidants. Both enzymes have been used in their native form to prevent oxidative damage but they have been found to be effective only following repeated doses or when administered locally or in isolated tissues or cells. Further, targeting and unsatisfactory pharmacokinetics have limited the use of these enzymes in methods of treatment (Abuchowski, et al. (1977) J. Biol. Chem. 252:3578-81; Monfardini and Veronese (1998) Bioconjug. Chem. 9:418-50).
As an alternative, several synthetic, free radical scavengers have been evaluated in animal models of cerebral ischemia and reperfusion and have been shown to be protective (Watanabe, et al. (1994) J. Pharmacol. Exp. Ther. 268:1597-604; Umemura, et al. (1994) Eur. J. Pharmacol. 251:69-74; Baker, et al. (1998) J. Pharmacol. Exp. Ther. 284:215-21; Itoh, et al. (1990) Psychopharmacology (Berl) 101:27-33; Gilgun-Sherki, et al. (2002) Pharmacol. Rev. 54:271-84). While some of the antioxidant-like SOD mimetics show efficacy in animal models, poor stability of these agents and the inability to sustain their retention at target sites are still major challenges for their therapeutic use. Delivery of SOD to the brain has been attempted via the use of prodrugs or carrier systems such as antibodies, liposomes (Kreuter (2001) Adv. Drug. Deliv. Rev. 47:65-81), and surface modifications such as conjugation to polyethylene glycol (SOD-PEG) (Veronese, et al. (2002) Adv. Drug Deliv. Rev. 54:587-606); however, treatment of cerebral ischemia/reperfusion injuries has been limited due to poor cerebral cell penetration. Gene therapy is an alternative approach but the delivery of genes into brain tissues and expression of therapeutic proteins may not be available for immediate effect (Hermann, et al. (2001) Neurobiol. Dis. 8:655-66). Furthermore, cerebral protein synthesis is severely compromised in injured areas after focal ischemia (Hermann, et al. (2001) supra; Hata, et al. (2000) J. Cereb. Blood Flow Metab. 20:937-46).
U.S. Pat. No. 6,123,956 teaches methods and compositions for treating stroke and/or traumatic brain injury. The compositions taught in this reference encompass a therapeutic agent such as SOD encapsulated in a pharmaceutically acceptable polymer, e.g., polyesters such as PLA (poly(lactide)) and PLGA (poly(D,L-lactide-co-glycolide)), polyethylene glycol, poloxomers, polyanhydrides, and pluronics), wherein the therapeutic agent is present at therapeutically effective concentrations which, if injected into the cerebrospinal fluid of a subject suffering from stroke or traumatic brain injury will contribute to the amelioration of the disorder.